1. Field of the Invention
The present invention relates to a light emitting diode device and manufacturing method thereof, and more particularly to a polychromatic light emitting diode device and manufacturing method thereof.
2. Description of the Related Art
Commercialized white light emitting diode devices, built using the present light emitting technology, produce white light by mixing of red, green and blue light, which are separately emitted from red, green and blue light emitting diodes. However, the method has a disadvantage of poor mixture quality. When white light emitting diode devices are used in a backlight source of a liquid crystal display, a diffusion plate and a brightness enhancement film can be utilized to enhance uniformity of mixing light. On the contrary, when the white light emitting devices are used for lighting, uniformly mixing light cannot be easily obtained. Moreover, the lifespan of red, green and blue light emitting diodes are different, and if any one fails, the color of light would become obviously imbalanced and harsh to users' eyes.
The current mainstream method uses light emitting diodes with phosphorus powders to mix white light. For example, blue light emitted from nitride based semiconductor light emitting diodes can be mixed with yellow light emitted from an excited yellow phosphorus powder to generate white light. However, the method has a disadvantage of the short lifespan of a yellow phosphorus powder, and especially, the yellow phosphorus powder is disposed so close to the light emitting diodes with high temperature such that its light conversion efficiency decreases beyond expectations. Moreover, phosphorus powders have an issue of low light conversion efficiency. However, the using an inorganic phosphorus powder for its longer lifespan would obtain light conversion efficiency that is lower than those of organic phosphorus powders. Therefore, various researches have been carried out to develop white light emitting diodes without using phosphor powers or to develop polychromatic light emitting diodes.
A paper titled “Monolithic Polychromatic Light-Emitting Diodes Based on InGaN Microfacet Quantum Wells toward Tailor-Made Solid-State Lighting,” Applied Physics Express 1 (2008)011106, discloses a method that uses silicon oxide stripes as a mask and epitaxially grows microstructured InGaN/GaN quantum wells on unmasked areas. Due to the alteration of growth conditions and mask geometry can be changed to emit various wavelengths, light of various wavelengths can be mixed to generate white light. A paper titled “Structural and Optical Properties of In-Rich InAlGaN Heterostructures for White Light Emission,” Japanese Journal of Applied Physics, Vol. 47, No. 6, 2008, p.p. 4413-4416, discloses a method that trimethylaluminium flow rate and reactor pressure are adjusted to form three-dimensional island structures during a metal organic chemical vapor deposition process on a InGaN layer. Due to low surface mobility of aluminum atoms separate to in-rich phase, a broad emission from green to red wavelengths can be observed. Combining blue emission from InGaN layer with green to red emission from In-rich InAlGaN alloy layer, white light emission has been obtained.
A paper titled “Phosphor-free white light-emitting diode with laterally distributed multiple quantum wells,” APPLIED PHYSICS LETTER 92, 091110 (2008), relates to a method that a portion of structure of blue InGaN multiple quantum wells is etched away, and green InGaN multiple quantum wells are epitaxially grown on the etched portion. Thus, the final structure can emit blue and green light. Another method, which is used to produce a multiple quantum well structure including different single quantum well layers each emitting corresponding blue or green light, is disclosed in a paper titled “Phosphor-Free GaN-Based Transverse Junction Light Emitting Diodes for the Generation of White Light,” IEEE PHOTOICS TECHNOLOGY LETTERS, VOL. 18, NO. 24, Dec. 15, 2006, U.S. Pat. Nos. 7,279,717, 7,042,017, 6,163,038, 7,361,937, 7,294,865, 7,279,716, and U.S. Patent Publication No. 2006/0,043,385, wherein U.S. Pat. No. 7,279,716 teaches using red phosphor to generate white light.
Moreover, several other methods are developed, and those includes a method, disclosed in U.S. Pat. No. 7,217,959, using a blue light emitting layer composed of quantum dots formed on an active layer emitting blue light; a method, disclosed in U.S. Pat. No. 7,271,417, of epitaxially forming a porous light-emitting layer, which can emit light with a plurality of wavelengths; and a method, disclosed in U.S. Patent Publication No. 2002/0,041,148, of epitaxially forming III-V semiconductor layer and II-V semiconductor layer to stack together, and emitting different wavelengths from them to mix a white light.
In all above exemplary prior arts, light emitting material emitting a second wavelength light is disposed between an n-type conductive layer and a p-type conductive layer; however, such a configuration may easily change the profile of a p-n junction, further affecting the light emission or electrical characteristics of the light emitting diode. In addition, some prior arts still need phosphor; however, red phosphor has low light conversion efficiency. Moreover, some prior arts need lithography and etching processes between two epitaxial growth processes, and the process steps are complex and may result in low yield.